As shown in FIG. 6, a Rzeppa type constant velocity universal joint which is a fixed type constant velocity universal joint includes an outer joint member 3, an inner joint member 6, balls 7, and a cage 8. The outer joint member 3 has a spherical inner surface 1 in which a plurality of track grooves 2 are formed in the circumferential direction at regular intervals and along the axial direction. The inner joint member 6 has a spherical outer surface 4 in which track grooves 5 opposite the track grooves 2 of the outer joint member 3 are formed in the circumferential direction at regular intervals and along the axial direction. The balls 7 for transmitting torque are provided between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6. The cage 8 for retaining the balls 7 is arranged between the spherical inner surface 1 of the outer joint member 3 and the spherical outer surface 4 of the inner joint member 6. The outer circumference of the outer joint member 3 and the outer circumference of a shaft coupled to the inner joint member 6 are covered by a boot to enclose grease as a lubricant in the joint (not shown).
As shown in FIG. 6, the center of curvature of the spherical inner surface 1 of the outer joint member 3 and the center of curvature of the spherical outer surface 4 of the inner joint member 6 are both formed at a center O of the joint. On the contrary, a center of curvature A of the track grooves 2 of the outer joint member 3 and a center of curvature B of the track grooves 5 of the inner joint member 6 are offset from the center O of the joint by an equal distance L in the axial direction. With this, when the joint forms an operating angle, the balls 7 are always guided onto a plane which bisects the angle formed between both axes of the outer joint member 3 and the inner joint member 6 to transmit rotational torque at an equal speed between two shafts.
By the way, in an automobile left in an extremely cold region for a long time, the fixed type constant velocity universal joint used for driving the front wheels has a low temperature (e.g., −20° C. or lower) equal to outside-air temperature, and grease enclosed in the fixed type constant velocity universal joint has the same low temperature.
In such a case, immediately after torque is inputted to the fixed type constant velocity universal joint at a high operating angle, stick slip can occur due to temporary lack of lubricity between the track grooves in the joint and the balls guided in the track grooves. When such stick slip occurs, the driver can hear abnormal noises. The abnormal noises are called abnormal noises at low temperatures. Since the abnormal noises at low temperatures disappear immediately when the joint is rotated to generate heat for increasing the temperature therein, the noises become a problem only when the vehicle left in a low temperature environment for a long time is driven and do not become a problem other than immediately after it is driven.
There has been a constant velocity universal joint which can prevent abnormal noises, not the abnormal noises at low temperatures, from occurring (Patent Documents 1 and 2). Patent Document 1 describes a plunging type constant velocity universal joint in which the center portions in the circumferential direction on pocket wall surfaces opposite in the joint axial direction of a cage pocket are ridged, with the ridged portions being as desired ball interference, thereby preventing the abnormal noises from occurring.
Patent Document 2 describes a fixed type constant velocity universal joint in which the clearance between a cage pocket and balls is 0 to 30 μm and grease having a friction coefficient of 0.13 or less is applied to track grooves, thereby preventing hammering sound from occurring.